Geoscience Reference
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phenotypic values for other traits and it is not proficient in deter-
mining the effects of QTLs (Tanksley 1993). In addition, only
interval mapping can be used for the QTL detection because
the phenotypic effects would be extremely overvalued in case
of single-point analysis (Lander and Botstein 1989).
Conventional
breeding and
marker-assisted
backcrossing
The trait governed by a single gene or by a gene that accounts
for a high proportion of the phenotypic variance could be sig-
nificantly transferred from donor to recipient line. Since long
time, traditional backcross breeding programmes were per-
formed for introgressing the qualitative traits on the assumption
that the proportion of the recurrent parent genome is recovered
at a rate of 1-(1/2)
t
+1
for each
t
generations of backcrossing.
Thus, 96.9% recovery of recurrent parent genome is expected
after four backcrosses. The deviation from this expectation is
entirely due to chance and linkage between the genes from
the donor parent being selected for with nearby genes. A good
example of the surprising amount of linkage drag that accom-
panies backcross breeding programmes was reported by Young
and Tanksley in 1989, who genotyped the chromosome carry-
ing the tomato mosaic virus (
Tm2
) disease-resistance gene in
several tomato cultivars that were developed by introgressing
the gene from a wild relative,
Lycopersicon peruvianum
via
backcross breeding. They found that even cultivars developed
after 20 backcrosses contained introgressed segments as large
as 4 cM and one cultivar developed after 11 backcrosses still
contained the entire chromosome arm carrying the gene from
the donor parent. A minimum of six backcross generations
would normally be required to recover 99% of the recurrent
parent genome, for the transfer of a single dominant gene. This
procedure is too lengthy, particularly in the perspective of the
competitive nature of modern hybrid breeding programmes,
where the turnover times for new lines and hybrids are fast.
Marker-assisted selection
Selecting a phenotype after
manipulating genomic regions that are involved in the appear-
ance of that phenotype through molecular marker is known
as marker-assisted selection. With the advent of an array of
molecular tools and techniques, and subsequently reasonably
dense molecular genetic maps in various crop plants, marker-
assisted selection has become feasible for traits both governed
by major genes and QTLs. It may greatly increase the efficiency
and effectiveness of selection in modern plant breeding through
the precise transfer of genomic regions of interest and by fast
recovery of recurrent parent genome compared to conventional
